The present invention relates to a primarily crystalline. titanium-containing protective layer of hard material having a homogeneous distribution of elements for the surface of a substrate material which can be highly stressed.
The protection of substrate materials by means of surface treatment or coating has gained in importance in light of the special requirements imposed by new and advanced technologies, such as high temperature materials, and due to the demand for a less abrasive material with longer edge life, for example, for metal cutting material processing.
Many conditions that are often difficult to satisfy at the same time must be considered with hard material coating.
The problem of finding a suitable hard material which can be used as a coating material is based on the fact that not all requirements can be met with a single hard material and that the required properties may even be in part mutually exclusive.
The constitution of the material system, the manufacturing parameters and the microstructure continue to play an important role in the coating of materials.
Hard materials fall into three groups based on their varying bonding structure:
(a) heteropolar hard materials (oxides of aluminum, zirconium, titanium and beryllium).
(b) covalent hard materials (borides, carbides, and nitrides of aluminum, silicon, boron and diamond).
(c) metallic hard materials (borides, carbides, and nitrides of the transition metals).
All of these hard materials melt at high temperatures.
In part their other properties vary noticeably.
Of course, the heteropolar hard materials are thermodynamically stable and do not interact appreciably with other materials. However, they are brittle and less hard. The thermal coefficient of expansion of the heteropolar hard materials is higher than that of the other hard materials.
The covalent hard materials exhibit high hardness, resistance to oxidation, low coefficient of expansion and low sensitivity to thermal shock. The covalent hard materials are especially advantageous because temperature has only a small influence on the mechanical properties of these materials.
Within this group of covalent hard materials, silicon carbide (SiC) and silicon nitride (Si.sub.3 N.sub.4) exhibit especially good properties.
Due to the inadequate bonding strength of the covalent hard materials on metal substrates, a narrow range has been placed on its application as a layer material.
The metallic hard materials form the most versatile layer materials.
Due to the high proportion of metal bonds, the metallic hard materials exhibit good adhesive properties. They are less brittle than the other hard materials and have a high electric and thermal conductivity.
A characteristic feature of the metallic hard materials is their frequently complete miscibility.
A protective layer of hard material must fulfill a number of conditions, which may in part be even mutually exclusive.
Thus, the protective layer on the substrate is supposed to have maximum adhesion to the substrate. On the other hand. however, the surface of the coating is to be as inert and oxidation resistant as possible and should not interact with other materials. The protective layer is supposed to be as hard as possible, but still be ductile and impact resistant. Moreover, the temperature coefficients of the protective layer and the substrate must be in the same range so that the protective layer does not flake off under thermal stress. Further, the properties of the protective layer should not change appreciably based on temperature.
Until recently no material has been found that fulfills all of these requirements as a coating material.
Therefore, composite protective layers of hard material that are constructed in a complicated manner are currently used.
Gradient layers can be manufactured with the metallic hard materials in which the composition changes continuously from the surface of the substrate to the surface of the protective layer. An especially adhesive material can be used to form a protective layer portion which adheres to the substrate while the surface of the protective layer comprises a metallic hard material having low tendency for interaction and consequently low adhesion to other materials.
Thus a part of the apparently mutually exclusive requirements can be fulfilled.
Even better results than with gradient layers can be obtained with multi-phased or multi-layered layers according to DE-PS 35 12 986. Such layers can, for example, be produced by sequential sputtering of two different cathodes.
Thus, a layer material having good adhesion properties can be selected as the first layer, and a compound having low adhesion property can be selected as the last, upper-most layer material. The sum of the hardness and the toughness may also be optimized by means of the good constitution of the intermediate interfaces. The formation of a plurality of interfacial junctions between mono-dispersive layers is important.
In their composition, multi-phased multi-layered and gradient layers must be precisely balanced with the properties of the substrate to be coated. The number of parameters to be considered increases thereby even further. Moreover, such a coating process is time-consuming to perform and consequently expensive.